Electron gun



May 4, 1937. Bfc. GARDNER ET AL.

ELECTRON GUN Filed May 31, 1932 lPatented May 4, 1937 .UNlTED'sTATi-:s PATENT oFFicE` ELECTRON GUN Bernard C. Gardner, Philadelphia, Pa., and Archibald HfBrolly, Palo Alto, Calif., as-

signors to Farnsworth Television Incorpo- A rated, a corporation of California v Application May 31, 1932, Serial No. 614,500

v 9 Claims.

Our invention relates tol electron guns or cathode ray projectors such as are used in4 oscillographs, and more particularly-to such cathode ray projectors as are used in oscillographs employed as receiving apparatus in television or the electrical projection of pictures.

Among the objects of our invention are: To provide an electron gun capable of producing an electron stream of high intensity; to provide an oscillograph cathode such that substantially all of the electrons emitted thereby enter the use- 'ful electron stream; to provide an oscillograph cathode which has a focusing action to concentrate substantially all of the emitted electrons at a common point; to provide an oscillograph cathode heated by alternating current; to provide an electron stream from an electron-emitting surface heated by alternating current unmodulated by the field produced by the heating current; and to provide an electron gun in which the interposition of a control electrode in a focused electron stream between anode and cathode does not materially change the degree of concentration.

Other objects of our invention will be apparent or will be specifically pointed out in the description forming a part of this specication, but we do not limit ourselves to the embodiment of the invention herein described, as various forms may be adopted withnthe scope ofv the claims.

Referring to the drawing:V

Figure 1 is a longitudinal sectional view of an oscillograph showing the electron gun in elevation.,

Figure 2 is an enlarged longitudinal section of the cathode, anode, and control electrode cornprising the electron gun as used in the Adevice of Figure 1.

In a device of the character described, the pri-v mary object is' to produce an electron stream, or pencil of cathode rays'whose impact on a luminescent screen gives rise to an intense and highly concentrated spot. vIn order to produce such an electron stream an electron gun is used; The conventional form of electron gun has a cathode composed of a thermionic filament, the electrons from which are drawn to a plate-shaped anode having an aperture through which a portion of the liberated electrons pass. The aperture is usually providedwith a nozzle or co-axial tube of narrow bore, the sides of which receive such electrons asv diverge from the axis of the stream at any material a',n`g1e.` v

'I'he pencil of cathode rays passing through the anode is "focused on the iiuorescent screen by proper adjustment of gas pressure, electrostatic (Cl. Z-27.5)

elds, or magnetic flelds/sharply localized in the plane of the electron gun, the eiort being to prevent divergence of the electrons Within the pencil.

These prior devices are relatively inemcient because ci the large number of the emitted electrons which are caught upon and held by the anode, and hence' never enter the useful field. Heretofcre, also larnentary cathodes have been commonly used, heated by the passageoi direct current.

Broadly speaking, our invention comprises the use of a concave emissive surface raised to emission temperature by an adjacent coil of wire heated by the passage of alternating current. The opening of a beam canal in a conical projec tion from an anode, is placed in the focus of the concave emitter so that practically the entire electron emission passes through the beam canal and is projected onwardly toward the luminescent screen. In that way We obtain large electron densities in the final beam. The concave emissive surface is placed exteriorly on a container completely surrounding and shielding the heater coil, the field of which is thereby prevented from modulating the emission, particularly when alternating current is used.

The Acontrol electrode acts also as a shield and has in it an aperture between the emissive surface and the opening to the beam canal. This aperture is traversed by grid wires which are seysectional view of the gun. Into one end of an envelope I of a heat-resisting glass such as pyrex, is sealed a reentrant stem 2. Mounted on .this stem is a clamp 4 to whichare weldedv screen supports 5 extending upwardly in the tube and welded securely to a screen ring 6 near the end of the envelope. These supports are preferably corrugated for stiffness and strength, and 4of a high resistance material such as nichrome or similar nickel-chromium-iron alloys.

Supported by the screen ring is a luminescent screen I which may be either of the well known fluorescent type, or a preferred type as described in the Vcopending application of -Philo T. Farnsworth, Serial'No. 655,784, filed Feb. 8, 1933.4

canal II. The taper of the beam canal may bc varied, but is preferably of an anglel so that a minimum of electrons can be picked up by its side lwalls and should correspond to the expansion of the electrons in the beam as it passes through,

Connection to a positive potential for the anode and screen ring is made by spring I2 which is sealed through the envelope at I4. The purpose of the use of resistance material in the screen supports 5 is to prevent the formation of eddy currents in the supports when magnetic means are used either to deflect or focus the beam.

Mounted on stem 2 is the cathode assembly which preferably comprises a container I6 of nickel or other suitable material having formed on its upper surface a spherically concave depression I1. The surface of the depression is preferably covered with an electron-emissive material I9, such as the rare-earth oxides having large emissivity at low temperatures, thus forming an exterior concave electron-emitting surface on the container I6. Inside the container I6 is a spiral refractory wire 20, one end of which is welded to the container at 2|, the other end being brought out through the hole 22 in an insulator 24 of refractory material such as lavite or isolantite. This insulator serves to position the spiral 20 in proper location within thc container so that all parts of the emissive surface I9 are equally heated.

In order that too great a heat may not be concentrated at the center of the emissive surface, the concavity oi the heating spiral is made greater than the concavity of the emissivc surface. Retaining flange 25 of metal is placed over the insulator, the edges of the container are then crimped over the retaining flange, thereby completely shielding the spiral-heater wire. A very small hole is left in the retaining flange for the end wire 2G to pass through, but as low voltages only are used on the heater coil only a. iev: thousandths of an inch clearance is necessary. .fi cathode support wire 2l is welded to the cathode and to a cathode lead wire 23, end wire 26 is welded to heater lead Wire 29, and both these lead wires are sealed through the reentrant stem 2.

The cathode is placed so that the focus of the concave emissive surfacev is just at the opening to the beam canal in the anode.

Interposed between the anode and cathode and surrounding the cathode is the control electrode 3D, preferably of thimble shape, the sides of the thimble extended downwardly below the cathode and acting as a shield.

The upper surface of the thimble is pierced by an aperture 3I of dimension smaller` than the emissive surface, preferably circular in shape and so positioned that all of the emission from the cathode emissive surface will pass through it. This aperture is traversed by grid wires 32 welded to the periphery of the aperture. As it is preferable to make these grid wires extremely fine and of a refractory metal such as tantalum or tungsten, it is desirable for purposes of Welding to place over them a nickel strip 34 and weld the nickel strip to the thimble; thus crushing and securely welding the refractory ne wires between the nickel strip and the thimble.

The grid wires 32 are shaped byl any convenient tool out of the plane of the aperture, and are disposed downwardly so that each wire describes an arc concave to the top of the thimble, and the section of a sphere described by the contours of all the Wires is concentric with the concave emissive surface. In this way minimum distortion of the electron stream is obtained when the control electrode is in operation.

Grid support wire 35 is welded to the skirt of the thimble and to grid lead 36 which is sealed into the stem 2. A V-shaped bend is preferably made in the grid support wire 35` so that the control electrode may be conveniently adjusted in relation to anode and cathode during assembly.

After all parts have been assembled as described, the envelope with' its contents is evacuated in accordance with approved methods known in the art, filled with gas if desired, and sealed ofi the pumps.

In operation, alternating current is applied to' heater coil 20, which, being of highly refractory metal, becomes heated and heats by conduction and radiation the cathode together with its concave emissive surface, which` when at the proper temperature, emits copious electrons. fact that the heater coil is entirely surrounded by the metal of the cathode container. none of the field produced by the alternation of current in the heater coil can affect the emission, and wc have i'ounr from moouiation as if direct current were used. but the :infuint of emission we obtain from an eX- icnded concave surfacel as described, is far greater y[han could be obtained from filaments as ordinarily used.

When 5-7000 volts positive potential are applied to the anode, the attraction of the end of the conc-shaped extension of the anode produces an intense electron concentration at the mouth or ine .roem canal, and due to the fact that all pciiions of the concave electron-emitting suri f L re cquidistant from the mouth of the canal, over per cent of all electrons emitted enter the canal and are projected through the canal io form an intense cathode ray beam on the other side. This beam may then be focused by any semble means to create a small spot on the .,crcen.

Beam currents of III milliamperes have been obtained in practice and it follows that the brilliant illumination of the screen so desirable in television is easily accomplished.

In order that picture frequencies may be used to control the intensity of thebeam without disturbing the concentration of the beam, the control grid wires are formed concentric with the curvature of the emitting surface. In this manner minimum distortion of the electron stream occurs, and modulation of the' beam takes place without materially disturbing the focus of the electrons at the point where they enter the beam canal. In other words, the percentage of electrons entering the beam canal is maintained substantially constant during the operation of the control grid over its operating range. This removes a common source of error in prior devices, and greatly aids in the production of an undistorted picture on the luminescent screen. In addition the grid thimble acts as an effective shield between cathode and anode and tends to stabilize the electrostatic elds. Such stabilization is desirable in order that stray fields from sources both inside and outside the tube may not change the direction or amount of emission before it` comes under the influence of the control element. It is particularly valuable in shielding the emission from the action of any deflecting means such as are commonly used, to bend the beam after it has passed through the beam canal.

Due to thc s;

yhat notl only is the emission as free ,We claim: f 1. An oscillograph cathode comprising a container having an exterior concave electron-emitting surface, and a concave spiral-heater disposed in said container substantially parallel to and extending across said aperture, the contour of each wire describing an arc of a circle convex toward the interior of said thimble.

3. A cathode-ray tube comprising an envelope, an indirectly heated cathode having an exterior -concave electron-emitting surface, a cylindrical hollow anode having an opening positioned at the focus of said emitting surface, and a control electrode shielding said cathode from said anode, said control electrode having an aperture -therein between said emitting surface and said anode, said aperture being traversed by grid wires formed substantially'concentric with said emitting surface. L4. A cathode ray tube comprising an envelope enclosing an electron-gun, said gun including an anode having a beam canal, acathode'having avconcave electron-emitting surface positioned`- 3 to focus an electron beam on the opening of said beam canal when a positive potential is applied to said anode, and control elements disposed in the space between anode and cathode following yan equipotential section of said beam.

35 5. A cathode-ray device comprising an envelope. and an anode, the combination with an electron-emitting cathode having a concave electron-emitting surface, of a control electrode comprising a plurality of wires disposed in a surface substantially concentric with the emitting surface of said cathode. r

6. A cathode-ray device comprising an enve- 'l lope. and an anode, thel combination with an electron-emitting cathode formed to direct the emitted electrons to a common center, of a control electrode comprising wires disposed in a geometrical surface substantially concentric with said center. l

7. A cathode-ray device comprising an envelope, and an anode, the combination with a cathode having an electron-emitting portion formed .to direct the emitted electrons to a common center, of a control electrode comprising a. thimble' surrounding said cathode and provided with an aperture over said electron-emitting portion, and having wires ytraversing said aperture welded to the edges of said aperture and disposed equidistant from said center..

8. A cathode ray tube having an envelope containing a cathode having an equipotential emitting surface formed to converge electrons to a. common center, and a central electrode comprising a thimble surrounding said cathode and provided with an aperture between said surface and said center, said aperture being smaller than said surface and larger than said center.

9. A cathode ray tube having an envelope containing a cathode having an equipotential emitting surface formed to converge electrons to a common center, and a. central electrode comprising a thimble surroundingsaid cathode and provided with an aperture between said surface and surface and larger than said center,.said aperture being of substantially the same diameter as the beam in the plane of the aperture.

BERNARD C. GARDNER. ARCHIBALD H. BROLLY.

said center, said aperture being smaller than said 

